postinstall-postinstall is unnecessary for anything in this project, I'm unsure why it's even in bluesky-social/social-app. The switch to pnpm means this package will never do anything, as its setup process only ever uses npm or yarn. Furthermore, if yarnpkg is in path, it will fail to install modules as it attempts to run the package manager yarn@pnpm@<version>, which doesn't work.
This package is hardcoded to work in very specific environments that can be easily identified by looking at its source code, making it worse than useless for this project.